broadcom/compiler: improve generation of if conditions

broadcom/compiler: improve generation of if conditions

Where it is safe to do so, avoid the generation of code to convert a
condition code into a boolean which is then tested to generate a
condition code. This is only done in uniform ifs, and only for condition
values that are SSA and only used once (in that if statement).

shader-db relative to MR 7726:

total instructions in shared programs: 8985667 -> 8974151 (-0.13%)
instructions in affected programs: 390140 -> 378624 (-2.95%)
helped: 810
HURT: 276
helped stats (abs) min: 1 max: 49 x̄: 17.77 x̃: 16
helped stats (rel) min: 0.10% max: 33.63% x̄: 7.97% x̃: 6.45%
HURT stats (abs)   min: 1 max: 46 x̄: 10.42 x̃: 10
HURT stats (rel)   min: 0.16% max: 21.54% x̄: 2.26% x̃: 2.03%
95% mean confidence interval for instructions value: -11.46 -9.75
95% mean confidence interval for instructions %-change: -5.76% -4.97%
Instructions are helped.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8709>